1. Field of the Invention
This invention generally relates to liquid crystal display (LCD) semiconductor fabrication and, more particularly, to a system and method for fabricating a substrate with a plurality of areas with different qualities, using a laser annealing process, with a corresponding plurality of masks.
2. Description of the Related Art
Polycrystalline silicon (poly-Si) material is used as the active layer of poly-Si thin film transistors (TFTs), for the fabrication of active-matrix (AM) backplanes. Such backplanes can be used to fabricate AM liquid-crystal displays and can be also combined with other display technologies, for example organic light-emitting diodes (OLEDs).
Poly-Si material is typically formed by the crystallization of initially deposited amorphous Si (a-Si) films. This process can be accomplished via solid-phase-crystallization (SPC), i.e., by annealing the a-Si films in a furnace at appropriate temperature, for a sufficiently long time. Alternatively, laser annealing can also be used to achieve the phase transformation.
Conventionally, all crystallization techniques are applied to a given substrate in such a manner as to yield poly-Si film of a uniform quality throughout the substrate area. In other words, there is no spatial quality differentiation over the area of the substrate. The most important reason for this end result is the inability of the current methods to achieve such quality differentiation. For example, when a-Si film is annealed in a furnace or by rapid-thermal-annealing, all of the layer will be exposed to the same temperature, resulting in the same quality of poly-Si material. In the case of conventional laser annealing, some differentiation is possible, but the price, in terms of loss of throughput, is very high for the modest performance gains. Hence, even for conventional laser annealing, such quality differentiation is not practically feasible.
Recently, a new laser annealing technique has been developed that allows for significant flexibility in the manner that the technique is applied and the resulting film microstructure. This technique relies on lateral growth of Si grains using very narrow laser beams, which are generated by passing a laser beam through a beam-shaping mask and projecting the image of the mask to the film that is being annealed. The method is dubbed Laser-Induced Lateral Growth (LILaC).
FIG. 1 illustrates steps in an LC-ELA annealing process (prior art). As seen in Step 1, initially amorphous silicon film is irradiated by a laser beam that is shaped by an appropriate mask to an array of narrow xe2x80x9cbeamletsxe2x80x9d. The shape of the beamlets can vary. In Steps 1-4, each beamlet is shaped as a straight slit of narrow width, approximately 3-5 microns (xcexcm). This slit is represented in the figure as the two heavy lines. The width of the slit is the distance between these two lines. This width can vary, but ultimately it is dependent upon the attainable lateral growth length (LGL), which is defined as the distance crystals can grow laterally (inwardly) from the edges of the irradiated area. Typically, the beamlet width is designed to be slightly less than twice the corresponding LGL.
The sequence of steps in FIG. 1 illustrates the growth of long polysilicon grains by LC-ELA process. A step-and-repeat approach is used. The laser beamlet width (indicated by the 2 parallel, heavy black lines) irradiates the film and, then steps a distance (d), smaller than half of the lateral growth length (L), i.e. d less than L/2. Using this step-and-repeat process, it is possible to continually grow crystal grains from the point of the initial irradiation, to the point where the irradiation steps cease. L is dependent upon a combination of film thickness and substrate temperature. For example, a typical value of L, for a 50 nanometer (nm)-thick film at room temperature, is approximately 1.2 microns (xcexcm). The value of L is due to this slow beamlet advancement. At each step, grains are allowed to grow laterally from the crystal seeds of the polycrystalline silicon (poly-Si) material formed in the previous step.
FIG. 2 is a plan view of a conventional optical system mask (prior art). The above-described process is equivalent to laterally xe2x80x9cpullingxe2x80x9d the crystals, as in zone-melting-crystallization (ZMR) method or other similar processes. As a result, the crystal tends to attain very high quality along the xe2x80x9cpullingxe2x80x9d direction, in other words, the direction of the advancing beamlets (shown by the arrow in FIG. 1). This process occurs in a parallel fashion (from each slit on the mask) allowing for rapid crystallization of the area covered by the projection of the mask on the substrate. Once this area is crystallized, the substrate moves to a new (unannealed) location and the process is repeated.
FIG. 3 is a pictorial representation of a system using the above-mentioned optical projection and the step-and repeat-process (prior art). Due to the step-and-repeat aspect of the laser projection, as opposed to furnace processes that treat a substrate uniformly, the LILaC process has the potential of permitting intentional spatial variations in the quality of the poly-Si material that is formed. Such intentional variations can be beneficial for applications where multiple components are integrated on an LCD display, where each component has different specifications and material performance requirements.
It would be advantageous if different areas of a substrate could be formed to different quality standards, to suit the function of the substrate area.
It would be advantageous if an entire substrate did not have to be annealed to meet the more stringent quality requirements of one particular area of the substrate.
It would be advantageous if a step-and-repeat laser annealing process could be used to anneal different areas of a substrate to different quality levels, according to need.
The present invention describes a process that yields poly-Si material on a given substrate having intentional variability in its crystalline quality. Furthermore, the process can precisely and reproducibly place p-Si material of a given quality to an exact location within the processed substrate.
Using variable poly-Si material quality on the same substrate enables monolithic integration of components, which have different material requirements, with simultaneous optimization of process throughput. Process throughout is inversely proportional to p-Si material quality. That is, low throughput corresponds to high quality. However, the poly-Si material quality and the area it occupies on a substrate are also inversely proportionally related (high quality to a small area). Hence, an opportunity exists to improve throughput and allow for integration of advanced components by utilizing LILaC technology in the manner that is described by the present invention.
Accordingly, a method is provided for fabricating variable quality substrate materials. The method comprises: selecting a first mask having a first mask pattern; projecting a laser beam through the first mask to anneal a first area of semiconductor substrate; creating a first condition in the first area of the semiconductor film; selecting a second mask having a second mask pattern; projecting the laser beam through the second mask to anneal a second area of the semiconductor film; and, creating a second condition in the second area of the semiconductor film, different than the first condition.
More specifically, when the substrate material is silicon, the first and second conditions concern the creation of crystalline material with a quantitative measure of lattice mismatch between adjacent crystal domains within their respective crystallized areas. For example, the lattice mismatch between crystal domains can be measured as a number of high-angle grain boundaries per area, where high-angle grain boundaries are boundaries separating adjacent crystal domains with a crystal lattice mismatch angle in the range between 15 and 90 degrees. To continue the example, forming a first number of high-angle grain boundaries per area in the first area may include forming adjacent high-angle grain boundaries separated by a first distance, while forming a second number of high-angle grain boundaries per area in the second area may include forming adjacent high-angle grain boundaries separated by a second distance, greater than the first distance.
The measurement of a distance between adjacent high-angle boundaries can be useful as a measurement of quality. For example, the second (greater) distance mentioned above may permit the complete formation of a transistor channel region in the second area between adjacent high-angle boundaries, that cannot be formed in the first area.
In some aspects, the method further comprises: selecting a plurality of masks; projecting the laser beam through each mask to anneal a corresponding area of semiconductor substrate; and, creating a particular condition in each area of the semiconductor film.
Additional details of the above-mentioned method, and a variably quality substrate are described below.